E-UTRAN and handover

ABSTRACT

The present invention relates to a method, a base station and an interface for handover in a wireless communication network. Handover is initiated when the signal quality falls below a predetermined value. Random access serves as an uplink procedure to enable the UE to make handover from a first base station (eNodeB) to a suitable second base station (eNodeB). In the present invention, random access parameters are exchanged between second and first base station before said first base station signals said random access parameters to said user equipment.

This application is the U.S. national phase of International Application No. PCT/SE2007/050542 filed 10 Aug. 2007, which designated the U.S., the entire contents of which is hereby incorporated by reference.

BACKGROUND

The technology disclosed herein relates to a method, base station, interface for handover in a wireless communication network which include at least a first base station (eNodeB) and at least one user equipment where handover initiates when the signal quality falls below a predetermined value and where random access parameters serves as an uplink procedure to enable the UE to make handover to a suitable second base station (eNodeB).

When a mobile phone is moving, it will be traveling through different cells. If the mobile phone is not engaged in a call, it will tell the network every now and then that it has moved to another cell. If the mobile phone is engaged in a call, the call of course needs to be maintained while the phone is moving. The process of replacing communication with one cellular radio station with another is called handover. When the cellular network sees the mobile phone moving closer and closer to another cell, it will initiate the handover process, during which the call will be transferred from one cellular radio station to another.

In WCDMA the handover access is done via dedicated channel. The dedicated channel uses closed loop fast power control. At the time of handover the RNC signals the initial UL DPCCH power setting parameter to the UE. This value is used by the UE to derive its initial uplink power for radio link that is to be added at the target cell. Since power control is in operation therefore uplink physical layer synchronization of the new radio link can be achieved quickly.

In the downlink of the mobile communication system LTE or E-UTRAN the modulation format is OFDM (Orthogonal Frequency Division Multiplex) for the signal bearer and the access scheme is OFDMA (Orthogonal Frequency Division Multiple Access). In the uplink of LTE, a slightly different multiple access scheme called single carrier frequency division multiple access (SC-FDMA) is used.

The actual implementation of the technology is different between the downlink (i.e. from base station to mobile) and the uplink (i.e. mobile to the base station) as a result of the different requirements between the two directions and the equipment at either end.

One fundamental difference between UTRAN and E-UTRAN in terms of handover procedure is that in the latter system the UE during the handover access the target cell via normal RACH channel [3]. In UTRAN the UE is assigned a dedicated channel during handover. This implies that in E-UTRAN it is important that the UE transmits RACH in the neighbour cell with the correct transmit power level and at the correct frame or sub-frame boundary. In order to allow UE to correctly estimate the RACH transmit power the source cell should provide power control and RACH format related parameters, which are applicable to the target cell. Furthermore these parameters should be conveyed to the UE prior to the initiation of handover.

There are two main issues in the existing system as discussed below:

In WCDMA the handover access is done via dedicated channel. The dedicated channel uses closed loop fast power control. At the time of handover the RNC signals the initial UL DPCCH power setting parameter to the UE. This value is used by the UE to derive its initial uplink power for radio link that is to be added at the target cell. Since power control is in operation therefore uplink physical layer synchronization of the new radio link can be achieved quickly.

In E-UTRAN Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink. OFDM is a modulation scheme in which the data to be transmitted is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks or units where a resource block is defined in both time and frequency. According to the current assumptions, a resource block size is 180 KHz and 0.5 ms in frequency and time domains, respectively. The overall uplink and downlink transmission bandwidth can be as large as 20 MHz.

For the LTE uplink, a different concept is used for the access technique. Although still using a form of OFDMA technology, the implementation is called Single Carrier Frequency Division Multiple Access (SC-FDMA). In essence a mobile is allocated a sub-carrier or sub-carriers for its link to the base station and it uses these to establish the uplink.

In E-UTRAN the handover access will take place via RACH, which cannot use and will not use fast power control. This means RACH transmission power during handover access should be as precise as possible. In general the handover access on RACH should use the correct setting, parameters and power levels. This requires some exchange of information related to RACH between eNode B. In the current WCDMA system no such information is exchanged. If such information is not exchanged between eNodeBs in LTE the uplink synchronization will be delayed. Secondly the collision on RACH may also increase. Thirdly the interference will increase due to increase in RACH load. Furthermore, if the serving and target cells use different RACH format then handover access to the target cell may completely fail if the UE accesses the target cell with the RACH format used in the serving cell. An alternative and straight forward solution is that prior to handover access the UE reads system information of the neighbour cells to acquire all RACH related information of the neighbour cell. The obvious disadvantage is that it will increase the handover delay and interruption time. Another problem is that it will increase complexity in the UE due to reading of neighbour cell information in RRC_CONNECTED mode.

SUMMARY

According to a first aspect of the technology disclosed herein, there is provided a method for handover in a wireless communication network which include at least a first base station (eNod B) and at least one user equipment where handover initiates when the signal quality falls below a predetermined value and where random access parameters serves as an uplink procedure to enable the UE to make handover to a suitable second base station (eNodeB).

-   -   said random access parameters exchanged between first and second         base station before said first base station signalling said         random access parameters to said user equipment (UE).     -   said signalling takes place before or at the time said user         equipment performs handover in connected mode to said second         base station.     -   said random access related parameters are exchanged over X2         interface between the base stations in LTE.     -   said wherein the said set of RACH related parameters exchanged         between the base stations correspond to RACH channel format,         RACH power control, RACH access control, RACH burst         construction.     -   said parameters correspond to Uplink interference, RACH         persistence level, backoff control parameters, SNR target level,         reference symbol transmitted power, Zadoff-Chu root sequence         index, cyclic shift information, high speed flag, time-frequency         allocation of RACH slot, offset for uplink and downlink         imbalance.

According to a second aspect of the technology disclosed herein, there is a base station (eNod B) communication with at least one user equipment where handover initiates when the signal quality falls below a predetermined value and where random access parameters serves as an uplink procedure to enable the UE to make handover to a suitable second base station (eNod B) where

-   -   said random access parameters transmits between first and second         base station before said first base station signalling said         random access parameters to said user equipment (UE).

According to a third aspect of the technology disclosed herein, there is a user equipment, in communication with a first base station, where handover initiates when the signal quality falls below a predetermined value and where random access parameters serves as an uplink procedure to enable the UE to make handover to a suitable second base station (eNod B) where

-   -   said random access parameters transmits between first and second         base station before said first base station signalling said         random access parameters to said user equipment (UE).

According to a third aspect of the technology disclosed herein, there is a interface (x2) between base stations in LTE where A user equipment, in communication with a first base station, where handover initiates when the signal quality falls below a predetermined value and where random access parameters serves as an uplink procedure to enable the UE to make handover to a suitable second base station (eNod B) where

-   -   said random access parameters transmits between first and second         base station before said first base station signalling said         random access parameters to said user equipment (UE).

Advantages of the technology disclosed herein is that we get an improved handover access via RACH and that Uplink synchronization via RACH to the target cell will be faster leading to shorter interruption time. UE does not have to read system information of the target cell to acquire RACH related parameters. This leads to less complexity in the UE. The idea of exchanging RACH related parameters and measured quantities is applicable to any other system where handover access to the destination cell takes place via RACH. However, some of the parameters to be exchanged would be specific to that particular technology.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the technology disclosed herein will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 illustrates the RACH related information to be exchanged over X2 interface (between eNode Bs)

FIG. 2 is a flowchart showing basic acts performed in a method of transmitting random access parameters between base stations before signaling the random access parameters to a user equipment for use in handover.

DETAILED DESCRIPTION

A glossary of the abbreviations used in this patent specification is set out below to facilitate an understanding of the technology disclosed herein.

-   -   E-UTRAN: Evolution UMTS Terrestrial Radio Access Network=LTE         (Long Term Evolution)     -   OFDM: Orthogonal Frequency Division Modulation     -   OFDMA: Orthogonal Frequency Division Multiple Access     -   RS: Reference Signal     -   AGW Access Gateway     -   eNode B Enhanced Node B     -   RACH Random Access Channel     -   SC-FDMA Single Carrier FDMA     -   UE User Equipment     -   SCH: Synchronization Signal     -   UE: User Equipment     -   CPICH: Common Pilot Indication Channel     -   FDD: Frequency division duplex     -   WCDMA: Wide band code division multiple access

FIG. 1 shows architecture of a radio network in which the technology disclosed herein may be implemented. An access gateway (aGW) is connected to several enhanced node Bs (eNode Bs). The interface between aGW and eNode B is called the S1 interface and the interface that exists between eNodes is called X2 interface. RACH related information is exchanged over the X2 interface of FIG. 1.

In a typical cellular radio system, a geographical area is divided into cell areas served by base stations which are connected to a radio network. Each user (mobile subscriber) in the cellular radio system is provided with a portable, hand-held, mobile station (user equipment unit or UE) which communicates voice and/or data with the mobile radio network. Each base station includes a plurality of channel units including a transmitter, a receiver, antennas etc. Each user equipment unit (UE) also includes a transmitter, a receiver, antenna, user interface and is identified by a specific user equipment unit (UE) identifier.

In a cellular radio communications system, a handover operation allows an established radio connection to continue when a mobile radio participating in that connection moves between cells in the system. Handover is typically initiated when the signal strength or signal quality of the radio connection with an origination base station falls below a predetermined threshold value. Often, a low signal strength or a poor signal quality indication means that the user equipment unit (UE) is near a border between the two cells. If the user equipment unit (UE) moves closer to a destination cell or to a clearer line of unobstructed sight, handover of the radio connection to the destination cell usually results in improved radio transmission and reception. Deciding which cells to involve in handover often requires coordination between the user equipment unit (UE) and the radio network. For example, in Wideband CDMA (WCDMA), the user equipment unit (UE) maintains a list of cells to monitor for potential handover purposes. The list of cells maintained by the user equipment unit (UE) includes cells comprising an “active list”, as well as adjacent cells which (although not in the active list) are to be monitored. The user equipment unit (UE) continuously updates its list of cells to monitor based on information communicated to the user equipment unit (UE) by the network. For example, the network may provide, via a message such as a measurement control message, an initial list of cells via a message such as a measurement control message. The initial list of cells may be, for example, a list of cells which neighbor the cell in which the user equipment unit (UE) is located. Thereafter the network may update the user equipment unit (UE) regarding what cells should be included in the active set using a message such as an active set update message.

The user equipment unit (UE) monitors, e.g., makes measurements regarding, the base station control or broadcast channel of each of the cells included in the list maintained by the user equipment unit (UE). The monitoring results (e.g., measurements) are transmitted to the network where, based on this monitoring, the network makes a determination as to what cells should be in the active set (e.g., what cells should be added, replaced, or removed).

Different types of control channels may exist between one of the base stations and user equipment units (UEs). For example, in the forward or downlink direction, there are several types of broadcast channels including a general broadcast channel (BCH), a paging channel (PCH), a common pilot channel (CPICH), and a forward access channel (FACH) for providing various other types of control messages to user equipment units (UEs). In the reverse or uplink direction, a random access channel (RACH) is employed by user equipment units (UEs) whenever access is desired to perform location registration, call origination, page response, and other types of access operations. The random access channel (RACH) is also used for carrying certain user data, e.g., best effort packet data for, e.g., web browser applications. Traffic channels (TCH) may be allocated to carry substantive call communications with a user equipment unit (UE).

When a UE shall make access to an E-UTRAN radio network, the UE seeks access to the network in order to register and commence services. The random access (RA) serves as an uplink control procedure to enable the UE to access the network.

Random access in E-UTRAN is based on a two-step procedure. In the first step the UE transmits a randomly selected signature to the network. In the subsequent procedure the network responds to the UE with an uplink scheduling grant which is used by the UE to transmit further details related to the connection request.

The transmitted sequence is randomly chosen out of an available pool of 64 unique sequences. If the UE does not receive an uplink scheduling grant within a certain time it randomly selects a new signature sequence and performs a new random access (RA) attempt.

The time-frequency resources where random access can be performed are announced via system information. One random access opportunity (or resource) is 1.07 MHz wide (6 resource blocks) and lasts either for 1 ms or 2 ms, depending on the RACH preamble format. Multiple RA opportunities may be spread out over frequency. It is up to the network whether to schedule other data in a RA slot or not. The network thus also controls whether RA transmission is orthogonal to shared data transmission or not.

RACH Format and Associated Parameters.

The RACH burst in LTE contains a cyclic prefix, the RACH preamble, and a guard interval. The cyclic prefix is in the beginning of the RACH burst and is a copy of the last part of the RACH preamble. The cyclic prefix enables efficient frequency-domain processing of the RACH burst in the eNode B RACH receiver. The guard interval accounts for the unknown round trip delay in the cell. Both cyclic prefix and guard interval must be larger then the maximum round trip delay to ensure proper operation.

The LTE standard defines 3 RACH preamble formats:

-   -   1. Standard format, 1 ms: The preamble part of the RACH burst is         not repeated. The cyclic prefix and guard period are balanced         and enable cell sizes of approximately 15 km (only considering         round trip delay, not link budget).     -   2. Format with extended cyclic prefix, 2 ms: This format         provides extended cyclic prefix and guard periods but no         repetition of the preamble. The cyclic prefix and guard period         are balanced and enable cell sizes of approximately 80 to 90 km         (only considering round trip delay, not link budget).     -   3. Repeated preamble format: The preamble is repeated to enable         a higher received energy at the receiver.

The RACH preamble is derived from Zadoff-Chu root sequences. These sequences have ideal periodic auto-correlation functions and given this auto-correlation also best possible periodic cross-correlation functions. Depending on the cell size multiple RACH preambles can be derived from a single Zadoff-Chu root sequence: In addition to the root Zadoff-Chu sequence—which is always a valid RACH preamble—additional preambles can be derived by cyclic shifting the Zadoff-Chu root sequence integer multiples of the minimum shift amount. This minimum shift amount depends on the cell size and must be at least as large as the maximum round trip delay plus maximum expected delay spread in the cell. This condition together with the ideal auto-correlation function insures that a RACH preamble transmitted with a certain cyclic shift never creates a correlation peak in a zone associated with another cyclic shift, all RACH preambles derived from a single Zadoff-Chu root sequence are orthogonal.

If the cell size becomes too large not all required 64 preambles can be derived from a single Zadoff-Chu root sequence, in this case additional root sequences needs to be allocated. Preambles derived from different root sequences are not mutual orthogonal.

Information that must be conveyed to a terminal is the Zadoff-Chu root sequence together with the minimum cyclic shift value. These two information elements enable a terminal to construct a RACH preamble. Even in case that a single Zadoff-Chu root sequence is not sufficient this information is sufficient since the terminal can calculated how many root sequences are needed and pick them according to a predefined order.

In case of cells where terminals with high speeds are expected not all cyclic shift values are allowed. In this case one-bit information is required to signal this.

The parameters describing the preamble (Zadoff-Chu root sequence, minimum cyclic shift value and high speed flag) together with the RACH format enable the terminal to construct the RACH burst. In addition to these parameters some additional information is required by the UE to know when and at which frequency to transmit the RACH burst, i.e. information regarding the allocation of the RACH slot is also needed.

RACH Configuration in Different Cells.

Each cell will use only one type of RACH format. The format selection is implementation dependent. Generally it would depend upon the cell size, desired coverage etc. However, adjacent cells may use different RACH formats since cell coverage may vary in the same deployment area. Secondly, the network planning can change in static or semi-dynamic manner, which would require change in the RACH format. The parameters associated with the RACH format shall be signaled in the cell via system information.

RACH Transmitted Power Estimation.

The open loop power control is used by the UE to estimate the power with which it transmits RACH pre-amble. The estimation is based on downlink path loss and uplink interference measured at the base station. The RACH preamble transmission power (P_(RAcH)) is described below:

Assume that a UE transmits with certain power level such that the signal received at the base station achieves certain SNR_(t), which can be expressed as follows in linear scale by (1):

$\begin{matrix} {{SNR}_{t} = \frac{P_{RACH} \times g_{UL}}{I_{UL} + N_{0}}} & (1) \end{matrix}$ g_(UL): is uplink path gain I_(UL): is uplink received interference

-   -   N₀: is white noise

The above equation can be expressed in dB scale as follows: P _(RACH)=SNR_(t)+(I _(UL) +N ₀)−g _(UL)[dBm]  (2)

In (2) all variables are unknown. The quantities SNR_(t) and I_(UL) can be signaled to the UE via system information. However, the third variable g_(UL) can be estimated from the downlink measurement quantity called reference symbol received power (RSRP), which is specified in E-UTRAN, as follows: RSRP=P _(RS) −g _(DL)[dBm]  (3)

Note (3) contains downlink path gain (g_(DL)), whereas the goal is to estimate the uplink path gain (g_(UL)); the above relation can be rewritten as follows: g _(DL) =P _(RS)−RSRP[dB]  (4) P_(RS): is reference symbol transmission power

The relation between uplink and downlink path gain can be expressed as follows by applying cell specific offset (δ) to account for uplink/downlink imbalance, feeder losses etc. Thus, UE can derive uplink path gain (g_(UL)) from the downlink path gain as follows: g _(UL) =P _(RS)−RSRP+δ[dB]  (5)

Combining (2) and (5) we can express the derived RACH transmitted power as follows: P _(RACH)=SNR_(t)+(I _(UL) +N ₀)−P _(RS)+RSRP−δ[dBm]  (6)

This is state of the art method of deriving RACH transmitted power. Similar method is also used in WCDMA.

It is evident from (6) that in order to facilitate the UE to derive initial RACH transmission power the network has to signal the following parameters:

-   -   Target signal to noise ratio (SNR_(t))     -   Uplink received interference (I_(UL))     -   Reference symbol transmitted power (P_(RS))     -   Cell specific offset (δ)

The derivation of initial RACH power based on signaled parameters is commonly referred to open loop power control.

The above parameters are cell specific and are signaled to the UE via system information in WCDMA. These information elements except cell specific offset are not exchanged between the cells in the state of the art technologies. As discussed below some of these parameters are to be exchanged over X2 interface (i.e. eNode B-eNode B interface) in E-UTRAN.

In a E-UTRAN network a UE shall receive a handover command via the serving cell. The command will indicate to the UE the target cell, where it should perform the handover. The UE sends RACH in the target cell. In order to attain uplink synchronization quickly the serving cell provides all necessary RACH related parameters in the handover command. The aim of this technology disclosed herein is to exchange some of these parameters between the eNodeBs, as shown in act 2-1 of FIG. 2. These parameters are cell specific. The signaling of these parameters to the neighbour eNode B enables each eNode B to be aware of the RACH related status, parameters and information in its neighbour cells. Act 2-2 of FIG. 2 shows subsequent signaling of the random access parameters from the source base station node to the user equipment unit.

This information can be classified into two main categories:

-   -   Dynamic information     -   Semi-static information

Dynamic Information to be Exchanged Over X2.

By dynamic information is meant information that can vary in the order of few 100 ms duration or shorter. This dynamic information can be signaled either periodically, when requested by the source cell or when decided by the destination cell. This dynamic information is mainly related to RACH power control but there can be other cases as well. Examples are shown in table 1.

Semi-Static Information to be Exchanged Over X2

Semi-Static type of information is exchanged when the RACH format is changed in the cell. This is either due to change in cell planning or because of any other purpose depending upon the implementation. But most common signalling scenarios would be when decided by the destination cell or when there is request from the source cell. In principle these parameters can also be signaled periodically. Examples of such information are shown in table 2.

Some of the information listed in table 2 could also be configured in each eNode B via operational and maintenance (O&M) or by any other propriety interface.

In addition the classification of information in table 1 and table 2 into dynamic or semi-static respectively is just one typical example. In practice this may be a matter of implementation in the network. For example in another implementation especially if uplink load is low the uplink interference may be exchanged in semi-static manner.

In order to ensure better interoperability between eNode B, which may be belong to different vendors, preferably these parameters are set by each eNode B itself and eventually communicated to its neighbours. This means preferably they should be signalled via X2 interface (i.e. interface between the eNode B).

The technology disclosed herein being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the technology disclosed herein, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claim.

TABLE 1 Dynamic information Information Element Purpose Uplink RACH power control interference RACH RACH access load/persistence probability

TABLE 2 Semi-static information Information Element Purpose RACH format Overall status (0, 1, 2) SNR_(t) Reference symbol RACH power control transmitted power Zadoff-Chu root RACH burst sequence index construction Cyclic shift RACH burst information construction High speed flag RACH burst construction Time-frequency RACH access allocation of RACH slot Offset for UL/DL RACH power control imbalance 

The invention claimed is:
 1. A method for handover in a wireless communication network which includes at least a first base station and at least one user equipment, the method comprising: initiating handover when signal quality falls below a predetermined value and where random access parameters serve as an uplink procedure to enable the user equipment to make handover to a suitable second base station; transmitting the random access parameters between the first base station and the second base station before the first base station signals the random access parameters to the user equipment; the random access parameters including parameters that enable the user equipment to construct a RACH preamble and a RACH format parameter, the random access parameters including RACH channel format, RACH power control, RACH access control, RACH burst construction.
 2. A method for handover in a wireless communication network as claimed in claim 1, further comprising signalling the random access parameters before or at the time said user equipment performs handover in connected mode to said second base station.
 3. A method for handover in a wireless communication network as claimed in claim 1, further comprising exchanging the random access parameters over an X2 interface between the first base station and the second base station in a Long Term Evolution (LTE) radio access network.
 4. A method for handover in a wireless communication network as claimed in claim 1, wherein the random access parameters comprise Uplink interference, RACH persistence level, backoff control parameters, SNR target level, reference symbol transmitted power, Zadoff-Chu root sequence index, cyclic shift information, high speed flag, time-frequency allocation of RACH slot, offset for uplink and downlink imbalance.
 5. A method for handover in a wireless communication network as claimed in claim 1, wherein the random parameters corresponding to a destination cell are signaled to the user equipment by a serving cell via a shared channel.
 6. A base station in communication with at least one user equipment configured to initiate handover of the user equipment to a second base station when signal quality falls below a predetermined value, the handover being initiated in an uplink procedure using random access parameters, and wherein the random access parameters are transmitted between the second base station before the first base station signals the random access parameters to the user equipment, the random access parameters including parameters that enable the user equipment to construct a RACH preamble and a RACH format parameter, the random access parameters including RACH channel format, RACH power control, RACH access control, RACH burst construction.
 7. A user equipment in communication with a first base station, the first base station being configured to participate in a handover from the first base station to a second base station when signal quality falls below a predetermined value, the handover involving an uplink procedure that uses random access parameters, the user equipment being configured to receive the random access parameters transmits from the first base station after the random access parameters have previously been transmitted from the first base station to the second base station, the random access parameters including parameters that enable the user equipment to construct a RACH preamble and a RACH format parameter, the random access parameters including RACH channel format, RACH power control, RACH access control, RACH burst construction.
 8. A communications interface between a first base station and a second base station of a Long Term Evolution (LTE) radio access network, the interface being configured, when a handover of a user equipment is to be initiated from the first base station to the second base station when signal quality falls below a predetermined value, the handover involving an uplink procedure that uses random access parameters, the interface being configured to transmit the random access parameters between the first base station and the second base station before said first base station signals the random access parameters to said user equipment, the random access parameters including parameters that enable the user equipment to construct a RACH preamble and a RACH format parameter, the random access parameters including RACH channel format, RACH power control, RACH access control, RACH burst construction.
 9. A method for handover in a wireless communication network as claimed in claim 1, wherein the random access parameters comprise information specifying when and at which frequency to transmit a RACH burst.
 10. The base station of claim 6, wherein the random access parameters comprise information specifying when and at which frequency to transmit a RACH burst.
 11. The user equipment of claim 7, wherein the random access parameters comprise information specifying when and at which frequency to transmit a RACH burst.
 12. The interface of claim 8, wherein the random access parameters comprise information specifying when and at which frequency to transmit a RACH burst. 